The invention relates to the field of optical correlation making it possible to obtain the correlation function of one image with another. For example such systems make it possible to recognize a graphic in a given motif.
A known optical correlator system is described in French Patent Application No. 2 648 947 published on May 5th 1981. A recording is made on a photosensitive support of a system of interference fringes representing the diffraction pattern obtained from two parallel coherent beams on whose path have been interposed two objects having non-uniform transparency after focusing by a lens. This photosensitive support is read by one of the beams and in the focal plane of a second lens is obtained an intensity distribution characteristic of the correlation product between the two objects. When a graphic is to be found in a given motif, the image obtained is formed by peaks indicating the presence and position of this graphic in the motif in question. In the patent application referred to above the photosensitive support is a continuously recyclable medium, i.e. which can be inscribed or written without development and which can be erased at random. However, in such a system there are parasitic phase distortions induced by the optical components and, for the introduction of data, by photographic transparencies or electro-optical transducers. It is also known to interpose on the propagation of the waves a transparency, whose phase characteristic permits a strict compensation of the incident wave surface distortions. The filter is positioned in the Fourier plane, so that it can still be used no matter what the translation of the tranparency in the object plane.
The Fourier transformer system according to the invention compensates these distortions in a simpler manner. This system uses a wave front conjugate to the incident wave front which, at any point, is isomorphic to the latter. This conjugate wave front is, by inverse return, modulated a second time by the object. However, as a result of the reverse path, there is a compensation of the modulation deformations of the outward path. The same applies regarding deformations due to abberations of the lens, i.e. they are compensated.
The optical correlator system including the Fourier transformer system allows a significant gain to be obtained on the signal to noise ratio of the correlation peak. The latter is made equivalent to that resulting from an incoherent illumination.
By displacing the point source along a line segment an incoherent integration takes place in the image plane of the coherent images, whose noises are decorrelated. An electro-optical cell also may be interposed on the passage of the beam, which permits a translation of the cell. The low frequencies of the spectrum of the object transparency are attenuated by merely chosing the ratio of the different beams. Thus, this system makes it possible to process in parallel and in real time a large amount of data with less expensive optical components.